1. Field of the Invention
The field of the present invention relates in general to plasma reactors and processes typically used to process semiconductor substrates or the like. More particularly, the field of the invention relates to a shielded plasma reactor in which the plasma is generated primarily by inductively coupled power.
2. Background
The era of microelectronics, and more specifically of Very Large Scale Integration, has been built in large part around vacuum processing of semiconductors to obtain Integrated Circuits. One of the contributors in this field of vacuum processing, which enabled the development and success of today""s semiconductor industry, is undoubtedly the outstanding parallel development of plasma tools and of related processing techniques. The plasma-based equipment market is a multi-billion dollar annual business, be it for thin film deposition, etching or even ion implantation.
The history of plasma etching is relatively short, starting only around twenty years ago. The first commercially available equipment featured a basic diode-like configuration. The plasma was generated in a vacuum chamber between two flat electrodes. One of the electrodes was typically used as a wafer susceptor, and was also usually connected to a high-frequency power supply. The other electrode was typically grounded, although in some configurations the second electrode was connected to a high frequency power supply instead of or in addition to the first electrode. This configuration was very successful at the time, but important technological limitations started to become evident in the early eighties. As integrated circuit geometry continued to shrink dramatically, the semiconductor industry sought to develop plasma equipment capable of producing dense plasmas with low energy ions. A dense plasma is important in order to achieve a high rate of processing, while low energy ions are important to avoid damaging small integrated circuit features which are susceptible to damage from bombardment of high energy ions. In order to produce a high density, low ion energy plasma, it is desirable to de-couple the control of plasma density from the control of ion energy in a plasma.
The first serious generation of decoupled, or advanced, plasma sources that appeared in the eighties relied on a microwave power source and featured a special magnet arrangement (technology known as Electronic Cyclotron Resonance, or ECR) capable of delivering a high plasma density without high energy ion bombardment of the semiconductor wafer. The usefulness of conventional ECR to address the evolving needs of the semiconductor industry is limited, however, mainly because of the complexity and very limited operating pressure range of conventional ECR.
A more promising approach has more recently emerged based upon the conventional Inductively Coupled Plasma, or ICP, which was originally invented at the end of the 19th century. Modem ICP sources, which have been improved and adapted for use in semiconductor processing, appear to provide a technology with the potential to meet the needs of the semiconductor industry well into the next decade. While ICP and ECR both provide high density, low ion energy plasmas, ICP allows a drastically wider pressure range to be used for processing. The pressure range for typical ICP sources ranges from about 0.5 mtorr to about 1 torr, whereas typical ECR sources are limited to an operating range of about 0.5 mtorr to about 5 mtorr. Consequently, ICP is suitable for a very wide range of applications that intrinsically require very different process pressuresxe2x80x94from low pressure, fine pattern anisotropic etching to high pressure, isotropic etching.
Nevertheless, some conventional ICP sources suffer from a disadvantage in that they are prone to generate a significant amount of energetic ions. This is caused by the fact that the induction coil used to inductively couple energy into the plasma also causes some capacitive coupling of electromagnetic energy between the metal coil and the plasma (a phenomenon called parasitic capacitive coupling of the ICP inductor). In a typical ICP reactor, an induction coil surrounds a plasma production chamber below which a semiconductor substrate is placed for processing. Radio frequency power is applied to the induction coil and thereby inductively coupled into the plasma production chamber. The inductively coupled power accelerates ions circumferentially in the plasma substantially parallel to the semiconductor substrate. While inductively coupled power from the induction coil tends to accelerate ions in a plane parallel to the semiconductor substrate, the parasitic capacitive coupling tends to accelerate ions radially outward from the plasma which causes high energy ions to bombard the semiconductor substrate below the plasma. This problem is described in detail in U.S. Pat. No. 5,534,231 the disclosure of which is hereby incorporated herein by reference in its entirety.
This parasitic capacitive coupling can be substantially blocked by using a split electrostatic shield, also known as a split Faraday shield, positioned between the induction coil and the dielectric plasma chamber wall. The shield substantially blocks capacitive coupling while allowing inductive coupling of power into the plasma. Conventional split electrostatic shields typically comprise metal plates or a metal cylinder forming longitudinal slits transverse to the induction coil. The metal body of the shield blocks capacitive coupling, while the slits allow inductively coupled power to penetrate the shield. The slits prevent circumferential current loops from forming in the shield which would otherwise substantially prevent the penetration of the inductive electric field. Such electrostatically shielded ICP reactors are described in U.S. Pat. Nos. 4,918,031, 5,234,529 and 5,534,231, each of which is hereby incorporated herein by reference in its entirety. The excellent performance of such electrostatically shielded ICP sources demonstrates the promising potential of this technology for use in future semiconductor plasma processing equipment. Nevertheless, conventional split electrostatic shields are not ideal for all plasma processing. The slits allow some capacitive coupling through the slits which may in turn cause non-uniform power deposition in the plasma. In addition, conventional electrostatic shields typically comprise relatively complex, bulky and expensive solid-metal structures that must be fitted and supported around the outside of the dielectric ICP chamber wall.
What is needed is an electrostatic shield for ICP semiconductor processing reactors which provides more uniform and continuous shielding around the plasma. What is also need is an electrostatic shield that provides improved shielding of capacitive electric fields while allowing virtually unimpeded penetration of inductive electric fields. Preferably such an improved shield will be inexpensive and easy to manufacture and deploy.
One aspect of the present invention provides a thin film electrostatic shield for an inductively coupled plasma source for use in plasma processing. The thin film electrostatic shield provides a thin layer of conductive material between a source of inductively coupled power and a plasma. The conductive material is capable of blocking a desired amount of parasitic capacitive coupling from the power source or other source of capacitive power. In particular embodiments, a continuous thin film electrostatic shield (without slits or gaps) may be used between the source of capacitive power and the plasma, providing virtually complete capacitive shielding of the plasma. In other embodiments, the shield may contain slits or gaps to allow a desired amount of capacitive coupling. The slits or gaps do not have to be arranged transverse to the induction electric field as with conventional split electrostatic shields. Rather gaps can be arranged in any variety of patterns. For instance, small holes or gaps may be uniformly distributed around the shield to allow more uniform capacitive power deposition in the plasma.
The electrostatic shield is preferably sufficiently thin, so as to allow the inductive electric field to penetrate the shield and sustain the plasma. Unlike conventional split electrostatic shields, a longitudinal slit in the shield is not required to prevent circumferential currents in the shield and to allow power to be inductively coupled into the plasma. Rather, the fact that the shield is very thin allows power to be inductively coupled into the plasma through the shield with very little power loss. The thickness of the shield may be configured to allow a desired amount of inductively coupled power to penetrate the shield, although typically the shield is sufficiently thin such that substantially all of the inductive power penetrates the shield.
It is an advantage of these and other aspects of the present invention that capacitive power may be almost completely blocked, while inductive power is allowed to penetrate the shield virtually unimpeded. In particular, a continuous shield may be used in various embodiments without substantially blocking inductively coupled power as would be the case with a conventional continuous thick metal shield. It is a further advantage that gaps in any variety of patterns may be provided in the shield to allow a desired amount of capacitive coupling. The gaps need not be arranged in any specific relation to the source of inductively coupled power. In addition, the thickness of the shield may be selected to allow a desired amount of inductive power to penetrate the shield.
Another aspect of the present invention provides for a thin electrostatic shield that is deposited on, or forms a portion of, a plasma chamber wall. In particular embodiments, a thin layer of conductive material may be deposited directly onto a non-conductive chamber wall comprising quartz or other material substantially inert to the plasma processing environment.
It is an advantage of these and other aspects of the present invention that electrostatic shielding may be provided without a separate, bulky and expensive metal split electrostatic shield. Rather, the electrostatic shielding may be provided directly as part of the chamber wall or induction coil without the requirement of a separate stand-alone structure external to the chamber wall.